1. Field of the Invention
The invention is concerned with the preparation of coating compositions from epoxy-based resins and their application by cathodic electrodeposition.
2. Description of the Prior Art
Electrodeposition has become an important method for the application of coatings over the last two decades and continues to grow in popularity because of its efficiency, uniformity and environmental acceptance. Cathodic electrodeposition has become dominant in areas where highly corrosion-resistant coatings are required, such as in primers for automobile bodies and parts. Epoxy based systems provide the best overall performance in this application and are widely used.
Cathodic electrodeposition resins based on conventional epoxies obtained by reacting liquid diglycidyl ethers of bisphenol A with bisphenol A to produce higher molecular weight epoxy resins have known disadvantages. Such products tend to have excessively high softening points resulting in poor flow out. In addition, such products require excessive amounts of solvent during their preparation. In order to improve flow, it has been proposed to modify such conventional epoxy resins by reaction with a diol in the presence of a tertiary amine catalyst. Thus, Bosso et al., U.S. Pat. No. 3,839,252, describes modification with polypropylene glycol. Marchetti et al., U.S. Pat. No. 3,947,339, teaches modification with polyesterdiols or polytetramethylene glycols. Wismer et al., U.S. Pat. No. 4,419,467, describes still another modification with diols derived from cyclic polyols reacted with ethylene oxide. These various modifications, however, also have disadvantages. Tertiary amines or strong bases are required to effect the reaction between the primary alcohols and the epoxy groups involved. Furthermore, these reactions require long reaction times and are subject to gellation because of competitive polymerization of the epoxy groups by the base catalyst. In addition epoxy resins containing low levels of chlorine are required to prevent deactivation of this catalyst.
Many coating formulations applied by electrodeposition include pigments to provide color, opacity, application, or film properties. U.S. Pat. No. 3,936,405, Sturni et al., describes pigment grinding vehicles especially useful in preparing stable, aqueous pigment dispersions for water-dispersible coating systems, particularly for application by electrodeposition. The final electrodepositable compositions, as described, contain the pigment dispersion and an ammonium or amine salt group solubilized cationic electrodepositable epoxy-containing vehicle resin and other ingredients typically used in electrodepositable compositions. Among the kinds of resins used are various polyepoxides such as polyglycidyl ethers of polyphenols, polyglycidyl ethers of polyhydric alcohols and polyepoxides having oxyalkylene groups in the epoxy molecule.
Moriarity et al., U.S. Pat. No. 4,432,850 discloses an aqueous dispersion of a blend of (A) an ungelled reaction product of a polyepoxide and a polyoxyalkylenepolyamine, which is then at least partially neutralized with acid to form cationic groups, and (B) an additional cationic resin different from (A). The resulting dispersion is applied by cathodic electrodeposition and is disclosed as providing high throw power and films which are better appearing, more flexible and more water-resistant.
Anderson et al. U.S. Pat. No. 4,575,523, discloses a film-forming resin composition which when combined with a crosslinking agent and solubilized, is capable of depositing high build coatings in cathodic electrodeposition processes. The resin is a reaction product of a modified epoxy formed by reacting a water-soluble or water-miscible polyol, an excess of polyamide, and an aliphatic monoepoxide.
The automobile industry still has needs in the areas of controlled film thickness. The ability to build thicker, uniform films which are smooth and free of defects will allow the elimination of an intermediate layer of paint known as a primer surface or spray primer, previously required to yield a sufficiently smooth surface for the topcoat. Such an elimination results in removal of one paint cycle and provides more efficient operations. Thickener electrocoat primers may also provide improved corrosion resistance.